Re: [PATCH v2 0/3] File Sealing & memfd_create()

[Jan Kara <jack@suse.cz>]

On Mon 19-05-14 13:44:25, David Herrmann wrote:
> Hi
> 
> On Thu, May 15, 2014 at 12:35 AM, Hugh Dickins <hughd@google.com> wrote:
> > The aspect which really worries me is this: the maintenance burden.
> > This approach would add some peculiar new code, introducing a rare
> > special case: which we might get right today, but will very easily
> > forget tomorrow when making some other changes to mm.  If we compile
> > a list of danger areas in mm, this would surely belong on that list.
> 
> I tried doing the page-replacement in the last 4 days, but honestly,
> it's far more complex than I thought. So if no-one more experienced
> with mm/ comes up with a simple implementation, I'll have to delay
> this for some more weeks.
> 
> However, I still wonder why we try to fix this as part of this
> patchset. Using FUSE, a DIRECT-IO call can be delayed for an arbitrary
> amount of time. Same is true for network block-devices, NFS, iscsi,
> maybe loop-devices, ... This means, _any_ once mapped page can be
> written to after an arbitrary delay. This can break any feature that
> makes FS objects read-only (remounting read-only, setting S_IMMUTABLE,
> sealing, ..).
> 
> Shouldn't we try to fix the _cause_ of this?
> 
> Isn't there a simple way to lock/mark/.. affected vmas in
> get_user_pages(_fast)() and release them once done? We could increase
> i_mmap_writable on all affected address_space and decrease it on
> release. This would at least prevent sealing and could be check on
> other operations, too (like setting S_IMMUTABLE).
> This should be as easy as checking page_mapping(page) != NULL and then
> adjusting ->i_mmap_writable in
> get_writable_user_pages/put_writable_user_pages, right?
  Doing this would be quite a bit of work. Currently references returned by
get_user_pages() are page references like any other and thus are released
by put_page() or similar. Now you would make them special and they need
special releasing and there are lots of places in kernel where
get_user_pages() is used that would need changing.

Another aspect is that it could have performance implications - if there
are several processes using get_user_pages[_fast]() on a file, they would
start contending on modifying i_mmap_writeable.

One somewhat crazy idea I have is that maybe we could delay unmapping of a
page if this was last VMA referencing it until all extra page references of
pages in there are dropped. That would make i_mmap_writeable reliable for
you and it would also close those races with remount. Hugh, do you think
this might be viable?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR